If you want italic bold sans serif, just change the first declaration into

\DeclareMathAlphabet{\mathbfsf}{\encodingdefault}{\sfdefault}{bx}{sl}

but keep in mind that the OT1 encoded Computer Modern Sans fonts don't have slanted bold face, so also

\usepackage[T1]{fontenc}

or

\usepackage{lmodern}

is required.

You should also consider using \bm of the bm package instead of \boldsymbol, so that \bm{\mathsf{T}} would be equivalent to the \mathbfsf{T} with the first declaration. However there is no predefined sans serif italic math alphabet. So, if you need also sans serif italic, a more economic setting would be